1. Field of the Invention
The present invention relates to a magnetic sensor such as a magnetoresistive effect element for reproducing information in a magnetic storage device, and more particularly, to a magnetoresistive effect element having a CPP (Current Perpendicular to Plane) structure that applies a sense current in a stacking direction of spin-valve films.
2. Description of the Related Art
With the rapid proliferation of the Internet, digital TV broadcasting and the like, needs for mass storage devices are rapidly increasing. In mass storage devices such as hard disk devices, recording density is significantly improved. Particularly, since the middle of 1990s, the recording density has been improved at 60-100% annual rates. The key technical progress areas during the above-mentioned period include an increase in coercive force and reduction of medium noise in magnetic disks and, as for magnetic heads, the development of spin-valve GMR heads as magnetic heads for reproduction.
A conventional spin-valve GMR head includes a CIP (Current In-Plane) structure, that is, a sense current is applied in a direction within a film surface of a spin-valve film, electrons are scatted in accordance with the relative angle of magnetization between a fixed magnetization layer and a free magnetization layer forming the spin-valve film, and the resistance value of the spin-valve film varies. Currently, a rate of resistance change of 15% and a recording density of approximately 50 Gbit/in2 have been achieved.
In order to achieve a recording density in the 100 Gbit/in2 range of the next generation, TMR (Tunnel Magneto Resistance Effect) heads and CPP (Current Perpendicular to Plane)-GMR heads have been studied. Although TMR heads include a high rate of resistance change, there is a disadvantage in that a resistance value RA is several Ω μm2, which is a high value. On the other hand, CPP-GMR heads include features such that the resistance value RA is 1 Ω μm2 or less, which is a moderate resistance value RA, and as the element size is reduced, the rate of resistance change is increased and the device output is increased. Thus, CPP-GMR heads are expected to serve as sensitive reproducing heads in high-density recording apparatuses.
However, CPP-GMR heads have a problem in that the rate of resistance change is low and the sensitivity of a magnetoresistive effect element is insufficient for achieving a recording density of 100 Gbit/in2 range.
For example, there is an approach of miniaturizing the element size in order to improve sensitivity. However, even if photolithography techniques are used, the element size is limited to about 100 nm×100 nm, and further significant miniaturization cannot be expected. Hence, it is impossible to improve the sensitivity of magnetoresistive effect elements.